The present invention relates to a fuel injection apparatus for supplying fuel to a combustion chamber of an internal combustion engine and, more particularly, to a fuel injection apparatus suitable for a diesel engine.
A fuel injection apparatus of this type comprises a fuel supply pump such as a fuel injection pump and a unit injector coupled to the pump. The unit injector, which receives fuel from the fuel supply pump, has an injection plunger driven synchronously with the engines operation, and an injection chamber defined by the injection plunger. Thus, the unit injector can high-pressurize fuel from the fuel injection pump so as to inject it into the combustion chamber. Since the fuel injection apparatus of this type can inject fuel at high pressure into the combustion chamber of the engine, it is very suitable for a diesel engine requiring high pressure fuel injection.
In the fuel injection apparatus applied to a diesel engine, high-pressure fuel must be injected into the combustion chamber, and the quantity and timing of the fuel injection must be controlled in accordance with engine operation conditions. For this reason, a control means for controlling the injection quantity and timing of fuel is adopted in the fuel injection apparatus. For example, a known control means comprises a relief path for coupling the injection chamber of the unit injector with a fuel tank, and a single solenoid valve provided in the relief path. With the control means, timing for pressurizing fuel in the injection chamber, i.e., an injection timing, can be controlled when the relief path is closed under the control of the solenoid valve. Conversely, when the opening of the relief path is controlled by the solenoid valve, an fule injection end timing, i.e., a fuel injection quantity, can be controlled. However, when the fuel injection timing and quantity are controlled by the above-mentioned single solenoid valve, a high fuel pressure applies pressure on the solenoid valve while the fuel injection apparatus is operative. For this reason, the solenoid valve is easily damaged by the high fuel pressure, adversely affecting the precision with which the open/close operation can be performed and, similarly, hindering the precision with which both fuel injection quantity and timing can be controlled. Yet, when the injection pressure of fuel is decreased to protect the solenoid valve, the desired high-pressure injection cannot be performed.